The present invention relates to upscaling (upsampling) digital images and video for presentation on a display.
The resolution of a typical liquid crystal display is approximately 720×480 which is on the order of standard definition television. The resolution of a typical very high resolution diagonal display is 4096×2160 and typically has viewing distances that are less than 2 picture heights. In order to display a lower resolution image on a higher resolution display, the lower resolution image is upscaled (upsampled). Large high resolution displays viewed at close viewing distances tend to have annoying artifacts as a result of upsampling.
The traditional approaches to upscaling generally result in the introduction of visible degradation into the image to be enlarged. The visual degradation is primarily due to several factors. A first factor is related to using inexpensive Linear Shift Invariant (LSI) filters to upsample the image. Such LSI filters remove or attenuate high spatial frequency components in the input image which have the visual effect of blurring the details and results in aliasing which tends to result in various artifacts.
A second factor is related to the introduction of moiré in patterned textures, and spurious patterns and jaggedness along the edges. This results, at least in part, from using inexpensive LSI filters. Larger filter kernels can reduce these artifacts but only at the cost of ringing artifacts around sharp edges within the image. Ringing artifacts are a limitation of upsampling techniques based on LSI filters.
A third factor is the blurring of the edges in the upsampled image. Classical upsampling techniques are unaware of the position of edges and contours within the incoming image. When the upsampling filter encounters a sharp edge contour it simply continues its convolution across the edge, combining image samples on both sides of the edge. The effect is a blurred edge in the upsampled image.
Various improvements to classical upscaling technology have been developed. One class of techniques are edge adaptive techniques that locate the edges and contours within the incoming image and control the filter process near the edges. These techniques can produce an upsampled image with sharp natural looking edge contours. However, edge adaptive upsampling techniques share some drawbacks with LSI filters. One drawback of edge adaptive techniques is that they tend to blur textures (e.g., skin or sand). Another drawback of edge adaptive techniques is that they can mistake non-edge features for edges as a result of edge classification. This introduces local spurious edges that are not in the original, and that are very easily observed as mistakes.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.